1. Field of the Invention
The present invention relates to a highly human- and environment-friendly ink fixing method that is excellent in energy efficiency, operability and ink-fixing property and has a simple process, to an ink fixing apparatus that is excellent in energy efficiency, operability and ink-fixing property and is a highly human- and environment-friendly, simple and compact apparatus, and to a printer equipped with the ink fixing apparatus.
2. Description of the Related Art
Conventionally, printers such as stencil printers and ink-jet printers have used UV curable inks for printing operations that require high ink fixing property, and printers that cure inks by irradiation with UV light are disclosed. Such printers using UV curable ink are capable of curing the ink that has been applied onto a paper sheet in a relatively short time by irradiating it with UV light, fixing the ink to the paper sheet and thus preventing the generation of so-called “offset” that occurs during a continuous run (see Japanese Patent Application Laid-Open (JP-A) No. 2001-171221).
In such a printer that uses UV curable ink, an ink fixing apparatus for applying UV light is arranged behind a printing unit, where a paper sheet supplied with UV curable ink, ejected from the printing unit, is transferred for the fixing of the ink to the paper sheet by irradiation with UV light.
This ink fixing apparatus, however, has a problem that it increases apparatus size and thus requires a very large installation area, because a number of mechanical elements or components are required, such as an air-cooling fan and/or a heat exhausting duct for cooling the heat from a UV lamp, a shutter mechanism for opening and closing the fixing apparatus as needed, a shielding plate for preventing leakage of UV light to the outside of the fixing apparatus, etc.
With respect to ink fixing property, for example, UV curable inks with low light transmittance (e.g., black ink) require higher curing energy for full ink fixing than those with high light transmittance (e.g., yellow and blue inks).
For this reason, the ink fixing apparatus undesirably requires, as a standard, large curing energy sufficient to cure and fix UV curable inks with low light transmittance (e.g., black ink) i.e., inks that are most difficult to be cured. This causes increases in power source cost and maintenance cost.
In addition, it is known that the UV curing reaction is easily inhibited by oxygen present in the air to cause reduction in curing characteristics. To solve this problem, JP-A No. 2004-136672 discloses an ink fixing apparatus equipped with (1) a fixing member that can admit light covering the ultraviolet spectrum, (2) a transferring member arranged so as to face the fixing unit, and (3) a pressuring unit for pressing the transferring member against the fixing member, wherein a printed recording medium having UV curable ink is held between the fixing and transferring members and thereby the UV curable ink on the recording medium is cured by irradiation with UV light by means of the fixing member. By pressing the recording medium against the fixing member using the pressuring unit, it is possible to achieve curing (fixing) of UV curable ink by UV light and improvement in the curing characteristics by oxygen blocking as well.
General UV curable inks are monomers or oligomers before they are polymerized (cured), which are skin irritants having a pungent odor; they may be harmful to the human body and accordingly, it is desirable that no uncured portions be produced.
Depending on the condition under which UC curable ink is irradiated with UV light, though, it may result in the generation of poorly-cured portions (i.e., non-fixed ink). If this non-fixed ink comes off the recording medium in the course of printing, it may result not only in poor image quality, but also in negative influences on the environment and/or a person who touches the resulting printed matter. Even trace amounts of non-fixed ink may affect the human body when UV curable ink is specifically used for the printing on food packages.
In addition to the foregoing ink fixing apparatus using UV curable ink, there is disclosed a curing unit that uses electron beam (EB) in order to make inks with high pigment concentration available and to increase the thickness of ink film in a case of partial printing, e.g., printing of marks or lines on a print medium. In contrast to UV light, the transmittance of electron beam is not dependent on the pigment's concentration. Accordingly, the electron beam has the advantage of being capable of providing highly concentrated printed or coated matter with high visibility.
An electron beam irradiation device, however, has a problem that it requires an acceleration voltage of as high as 150 kV or more, causing the electron beam to directly act on a base material or even on a member under the base material to reduce physical its properties.
In a case of partial printing where the surface of a print medium is not necessarily patterned entirely (e.g., printing of marks and/or lines), it is ideal that only ink-supplied portions are irradiated with electron beams. In order to achieve this with a conventional electron beam irradiation device, operations electron beam's operations (e.g., electron beam focusing) are required, thus requiring a larger electron beam irradiation device. Such an irradiation device is, however, difficult to manufacture and irradiation of relatively small targets (e.g., marks and lines) with electron beams has been inefficient.
In addition, proposals have been made to perform plasma treatment on the surface of a target for the purpose of improving adhesion and surface modification of the target (see Japanese Patent (JP-B) No. 3040358, JP-A No. 2002-58995, and JP-A No. 2004-103423). For example, JP-B No. 3040358 discloses a method in which a solid dielectric is provided to at least one of electrodes, and an electrical field of 1 to 100 kV/cm intensity is applied for glow discharge plasma treatment under pressure close to atmospheric pressure.
JP-A No. 2002-58995 discloses plasma treatment in which the waveform of voltage to be applied across electrodes is changed to an alternating current voltage waveform with no idle time and in which the rise time of this alternating current voltage waveform is set to 100 μsec or less.
Moreover, JP-A No. 2004-103423 discloses plasma treatment in which discharge is allowed to occur in a discharge space under pressure close to atmospheric pressure by applying either a pulse wave of alternating positive and negative components or an alternating current voltage to each of a pair of electrodes at the same time and by causing the two waves to be in phase with each other with different polarities.
These plasma treatments, however, require complicated, troublesome adjustments for the production of an uniform glow discharge plasma and thus cause increase in the apparatus size. In addition, these patent literatures disclose the usage of plasma treatment for the improvement of film adhesion and film deposition, but never suggest or disclose their usage for ink fixing in a printer.
Thus the inventions that are disclosed in JP-B No. 3040358, JP-A No. 2002-58995 and JP-A No. 2004-103423 entail difficulty in the adjustment of voltage and are more likely to cause inefficiency due to concentration of discharge, resulting non-uniform energy rays on a target. Moreover, desirable ink fixing property may not be obtained when these inventions are applied to an ink fixing process.
Meanwhile, a technology has been disclosed in which print ink that contains a photoactivatable compound or photopolymerization initiator (e.g., photoinitiator, photoactivatable acid, or photoactivatable base) is cured in a plasma discharge chamber (see JP-A No. 2005-523803).
In this case, however, it is essential that the foregoing photoactivatable compound (photopolymerization initiator) be used as an ink ingredient to cure the print ink. In addition, a special reduced pressure container is required (preferably a reduced pressure container equipped with an inlet and an outlet), and the container needs to be filled with a particular gas such as nitrogen gas, helium gas, argon gas, neon gas, krypton gas, or xenon gas. Also, the ink curing needs to be carried out under a strict condition for excellent effects, making a fixing apparatus and/or fixing steps so complicated that it may result in increased costs. Since a special synthesized chemical, a photoactivatable compound, needs to be added to the print ink, there is a problem that ink cost will increase. Furthermore, such a photoactivatable compound contains a skin irritant, and in the event that non-fixed ink is remained on a printed matter as described above, it affects the human body when brought in contact with the skin via the printed matter. When the content of the photoactivatable compound is increased for improved ink fixing property, the ink sensitivity increases so much that curing reaction will proceed even by irradiation with a low-level light other than fixing light (e.g., cosmic rays such as α-ray), thereby causing stability problems (e.g., reduced storage stability, and ink fixation to the printer components).
There have been no technologies regarding to print inks containing no photoactivatable compounds, to an electrode structure that uses, without the need of pressure control, a plasma in an atmospheric pressure space for the curing (fixing) of ink, and to an optimal peripheral system, including a transferring unit required by the electrode structure and a unit for removing by-product gases.
Accordingly, a highly human- and environment-friendly ink fixing method that is excellent in energy efficiency, operability and ink-fixing property and has a simple process, an ink fixing apparatus that is excellent in energy efficiency, operability and ink-fixing property and is a highly human- and environment-friendly, simple and compact apparatus, and a printer that is equipped with the ink fixing apparatus and can provide high-quality images at low cost have not been provided; at present, the prompt provision of them has been demanded.